Our family Grandmother-Maurine- Beat her tomato plants with a stick after she watched the plants take off after a beating from a summer hail storm. I have learned not to doubt Maurine because she DID gardening, making her own soap, quilting, canning etc. etc.
Since I'm not into beating my tomatoes though-I decided last year to begin to prune them. I just went at them and it worked. This year I'm actually being more intentional with it but here is a good article that I want to share.
http://www.finegardening.com/how-to/articles/pruning-tomatoes.aspx